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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Interaction of surface radiation with convection in crystal growth by physical vapor transport

Growth of single crystals from vapor in closed ampoules is governed by an intricate interplay between mass, momentum and heat transfer processes. The objective of this study is to examine and isolate the effects of surface radiation heat transfer on the vapor transport process using a mathematical model. The model consists of a set of coupled nonlinear partial differential equations for conservation of mass, momentum, energy and species, and the integrodifferential equations which represent radiative exchange. It depends on five important physical parameters. These are Grashof number, Prandtl number, Schmidt number, aspect ratio and the radiation-conduction number. The effects of these dimensionless groupings are systematically investigated. From the cases examined, it is concluded that surface radiation can change the flow structure appreciably. This is especially true in microgravity environment where radiation competes primarily with conduction in modifying the thermal profiles. The numerical results also show that in the presence of radiation, the top heating configuration (source on top) is no longer stable and that near the growing crystal, radiation-induced vortices can introduce significant nonuniformities in the growth flux.

Kassemi, Mohammad↗
NASA NTRS1991

Growth kinetics of physical vapor transport processes: Crystal growth of the optoelectronic material mercurous chloride

Physical vapor transport processes were studied for the purpose of identifying the magnitude of convective effects on the crystal growth process. The effects of convection on crystal quality were were studied by varying the aspect ratio and those thermal conditions which ultimately affect thermal convection during physical vapor transport. An important outcome of the present study was the observation that the convection growth rate increased up to a certain value and then dropped to a constant value for high aspect ratios. This indicated that a very complex transport had occurred which could not be explained by linear stability theory. Better quality crystals grown at a low Rayleigh number confirmed that improved properties are possible in convectionless environments.

Singh, N. B.↗
NASA NTRS1999

Point Defect Distributions in ZnSe Crystals: Effects of Gravity Vector Orientation during Physical Vapor Transport Growth

ZnSe crystals were grown by the physical vapor transport technique under horizontal and vertical (stabilized and destabilized) configurations. Secondary ion mass spectroscopy and photoluminescence measurements were performed on the ZnSe samples to map the distributions of [Si], [Fe], [Cu], [Al] and [Li or Na] impurities as well as Zn vacancy, [V(zn)]. The annealings of ZnSe under controlled Zn pressures were studied to correlate the measured photoluminescence emission intensity to the equilibrium Zn partial pressure. In the horizontal grown crystals the segregations of [Si], [Fe], [Al] and [V(zn)] along the gravity vector direction were observed whereas in the vertically stabilized grown crystal the segregations of these point defects were radially symmetrical. No apparent pattern was observed on the measured distributions in the vertically destabilized grown crystal. The observed segregations in the three growth configurations were interpreted based on the possible buoyancy-driven convection in the vapor phase.

Su, Ching-Hua↗
NASA NTRS1999

Point Defect Distributions in ZnSe Crystals: Effects of Gravity Vector Orientation During Physical Vapor Transport Growth

ZnSe crystals were grown by the physical vapor transport technique under horizontal and vertical (stabilized and destabilized) configurations. Secondary ion mass spectroscopy and photoluminescence measurements were performed on the grown ZnSe samples to map the distributions of [Si], [Fe], [Cu], [Al] and [Li or Na] impurities as well as Zn vacancy, [V (sub Zn)]. Annealings of ZnSe under controlled Zn pressures were studied to correlate the measured photoluminescence emission intensity to the equilibrium Zn partial pressure. In the horizontal grown crystals the segregations of [Si], [Fe], [Al] and [V (sub Zn)] were observed along the gravity vector direction whereas in the vertically stabilized grown crystal the segregation of these point defects was radially symmetrical. No apparent pattern was observed on the measured distributions in the vertically destabilized grown crystal. The observed segregations in the three growth configurations were interpreted based on the possible buoyancy-driven convection in the vapor phase.

Su, Ching-Hua↗
NASA NTRS2001

Crystal Growth by Physical Vapor Transport: Experiments and Simulation Dynamics

Crystal growth from the vapor phase has various advantages over melt growth. The main advantage is from a lower processing temperature, which makes the process more amenable in instances where the melting temperature of the crystal is high. Other benefits stem from the inherent purification mechanism in the process due to differences in the vapor pressures of the native elements and impurities, and the enhanced interfacial morphological stability during the growth process. Further, the implementation of PVT growth in closed ampoules affords experimental simplicity with minimal needs for complex process control, which makes it an ideal candidate for space investigations in systems where gravity tends to have undesirable effects on the growth process. Bulk growth of wide band gap II-VI semiconductors by PVT has been developed and refined over the past several years at NASA MSFC. A new modeling approach for PVT has also been recently formulated and its validation and testing is the main objective of this work.

Ramachandran, N.↗
NASA NTRS1982

Gravity-driven convection studies in compound semiconductor crystal growth by physical vapor transport

Experimental results are summarized, and it is pointed out that gravity-driven convection can alter the diffusive-advective mass transport behavior in the growth of crystals by physical vapor transport. Specially designed and constructed transparent furnaces are described which are being used to study the effects of gravity in the crystal growth of the compound semiconductors PbTe and CdTe. The theory underlying vapor transport behavior is reviewed, with attention given to the vapor-solid behavior of compound materials, to one-dimensional mass transport, and to gravity-induced (natural) convection. In the transparent furnaces, the quartz capillary tube mounted along the axis of the main quartz ampoule is used to measure the temperature at the growth surface (vapor-solid crystal interface) and the source, as well as the complete temperature profile along the axis of the tube. The light-pipe works to remove heat from the growth end of the ampoule by radiative heat transfer. The ampoules are sealed after being evacuated to the low 10 to the -8th torr range with a cryogenic vacuum pump.

Zoutendyk, J. A.↗
NASA NTRS1989

Evaluation of transport conditions during physical vapor transport growth of opto-electronic crystals

Transport conditions were evaluated during the vapor phase growth of mercurous chloride crystals in a closed tube. Experimentally observed growth rates were much smaller than those calculated by the Hertz-Knudsen (H-K) equation. The Arrhenius behavior of growth rate with the temperature was used to derive the sticking coefficient. A one-dimensional diffusion model was used to calculate the total mass flux and was compared with the condensing flux. It was predicted that growth occurred in the convecto-diffusive range.

Singh, N. B.↗
NASA NTRS1998

Crystal Growth of ZnSe by Physical Vapor Transport: A Modeling Study

Crystal growth from the vapor phase has various advantages over melt growth. The main advantage is from a lower processing temperature which makes the process more amenable in instances where the melting temperature of the crystal is high. Other benefits stem from the inherent purification mechanism in the process due to differences in the vapor pressures of the native elements and impurities, and the enhanced interfacial morphological stability during the growth process. Further, the implementation of Physical Vapor Transport (PVT) growth in closed ampoules affords experimental simplicity with minimal needs for complex process control which makes it an ideal candidate for space investigations in systems where gravity tends to have undesirable effects on the growth process. Bulk growth of wide band gap II-VI semiconductors by physical vapor transport has been developed and refined over the past several years at NASA MSFC. Results from a modeling study of PVT crystal growth of ZnSe arc reported in this paper. The PVI process is numerically investigated using both two-dimensional and fully three-dimensional formulation of the governing equations and associated boundary conditions. Both the incompressible Boussinesq approximation and the compressible model are tested to determine the influence of gravity on the process and to discern the differences between the two approaches. The influence of a residual gas is included in the models. The preliminary results show that both the incompressible and compressible approximations provide comparable results and the presence of a residual gas tends to measurably reduce the mass flux in the system. Detailed flow, thermal and concentration profiles will be provided in the final manuscript along with computed heat and mass transfer rates. Comparisons with the 1-D model will also be provided.

Ramachandran, Narayanan↗
NASA NTRS1989

Ground-based simulation of telepresence for materials science experiments

A series of ground-based telepresence experiments have been performed to determine the minimum video frame rate and resolution required for the successive performance of materials science experiments in space. The approach used is to simulate transmission between earth and space station with transmission between laboratories on earth. The experiments include isothermal dendrite growth, physical vapor transport, and glass melting. Modifications of existing apparatus, software developed, and the establishment of an inhouse network are reviewed.

Johnston, James C.↗
NASA NTRS1998

Experimental Study of the Low Supersaturation Nucleation in Crystal Growth by Contactless Physical Vapor Transport

The process of the development of the nuclei and of subsequent seeding in 'contactless' physical vapor transport is investigated experimentally. Consecutive stages of the Low Supersaturation Nucleation in 'contactless' geometry for growth of CdTe crystals from the vapor are shown. The effects of the temperature field, geometry of the system, and experimental procedures on the process are presented and discussed. The experimental results are found to be consistent with our earlier numerical modeling results.

Grasza, K.↗
NASA NTRS1995

Growth of zinc selenide crystals by physical vapor transport in microgravity

The growth of single crystals of zinc selenide was carried out by both closed ampoule physical vapor transport and effusive ampoule physical vapor transport (EAPVT). The latter technique was shown to be a much more efficient method for the seeded growth of zinc selenide, resulting in higher transport rates. Furthermore, EAPVT work on CdTe has shown that growth onto /n11/ seeds is advantageous for obtaining reduced twinning and defect densities in II-VI sphalerite materials.

Rosenberger, Franz↗
NASA NTRS1987

Physical vapor transport crystal growth

The goals of this research are two-fold: to study effective means of growing ZnSe crystals of good optical quality and to determine the advantages of growing such crystals in microgravity. As of this date the optimal conditions for crystal growth have not been determined. However, successful growth runs were made in two furnances and the results are given.

Yoel, Dave W.↗
NASA NTRS1990

The growth of zinc selenide single crystals by physical vapor transport in microgravity

Growth and characterization studies will be performed on zinc selenide single crystals. The high temperature outgassing behavior of the silica ampoule material will be studied in order to develop a cleaning and bake-out procedure that will minimize the amount of impurities introduced into the vapor from the ampoule materials and in particular during the seal-off procedure. The outgassing behavior of the ZnSe starting material will be studied during high vacuum refinement at elevated temperatures in order to develop a temperature pressure program that will optimize the removal of impurities while minimizing a shift in stoichiometry due to preferred evaporation of the higher fugacity component. The mass spectrometer system was completed, and after calibration, will be used to perform the above tasks. The system and its operation is described in detail.

Anderson, Elmer E.↗
NASA NTRS2015

Flight Experiments of Physical Vapor Transport of ZnSe: Growth of Crystals in Various Convective Conditions

A low gravity material experiment will be performed in the Material Science Research Rack (MSRR) on International Space Station (ISS). The flight experiment will conduct crystal growths of ZnSe and related ternary compounds, such as ZnSeS and ZnSeTe, by physical vapor transport (PVT). The main objective of the project is to determine the relative contributions of gravity-driven fluid flows to the compositional distribution, incorporation of impurities and defects, and deviation from stoichiometry observed in the grown crystals as results of buoyancy-driven convection and growth interface fluctuations caused by irregular fluid-flows on Earth. The investigation consists of extensive ground-based experimental and theoretical research efforts and concurrent flight experimentation. The objectives of the ground-based studies are (1) obtain the experimental data and conduct the analyses required to define the optimum growth parameters for the flight experiments, (2) perfect various characterization techniques to establish the standard procedure for material characterization, (3) quantitatively establish the characteristics of the crystals grown on Earth as a basis for subsequent comparative evaluations of the crystals grown in a low-gravity environment and (4) develop theoretical and analytical methods required for such evaluations. ZnSe and related ternary compounds have been grown by vapor transport technique with real time in-situ non-invasive monitoring techniques. The grown crystals have been characterized extensively by various techniques to correlate the grown crystal properties with the growth conditions. This talk will focus on the ground-based studies on the PVT crystal growth of ZnSe and related ternary compounds, especially the effects of different growth orientations related to gravity direction on the grown crystals.

Su, Ching-Hua↗